The present invention relates to techniques for providing the transmission and reception of information and, more particularly, to digital transmission and receiving systems for communicating digitized voice or data information.
With recent advances in the communications art, there has been a trend to provide communication of information between transmitting and receiving systems of both voice and data information using digital techniques. In such systems, voice or data information is first digitized and then transmitted in a digital format to a receiver where it is then demodulated and decoded to convert the digital information to the original voice or data information. In systems operating at a predetermined carrier frequency, reception of such digitized information is not a problem since a receiver may tune to the carrier frequency, and thereafter receive and demodulate the transmitted digital information. In such systems, there is no need to synchronize the transmitter and receiver since the carrier is fixed and the data is being transmitted over the known carrier.
In communication systems which do not utilize a fixed frequency for data transmission, such as a frequency hopping system, where the frequency of transmission changes in a pseudorandom fashion, there is a need for synchronization between the transmitter of the digital data and the user or receiver receiving that digital data. In conventional frequency hopping systems, such synchronization has been typically accomplished by transmitting a time reference on a fixed frequency so the receiver can synchronize its internal clock with the internal time clock of the transmitter and thereafter receive data, including a synchronization preamble, which contains information sufficient to allow the receiver to synchronize to the frequency hopping pattern of the transmitter. This synchronization preamble has typically been provided only once in a message transmission, at the beginning of a message, so that synchronization must be achieved at that time or the message will be lost.
In accordance with the above technique, various problems have been encountered during the transmission and reception of information. In particular, if a receiving station is transmitting during the time that another transmitting station begins to transmit its information, the synchronization preamble from the other transmitting station will not be detected by the receiving station and will be lost. Thus, the receiving station will be unable to receive and decode the message unless the other transmitting station transmits the same message again. Likewise, if a signal is being received by a receiving station and that transmitted signal fades or is lost for some other reason due to interference (or otherwise) during message transmission, the receiver has no ability to reacquire synchronization without the transmitter reinitiating the same message transmission at another time. Each of the above are operational effects which limit the efficient use of any communication system and decrease reliability where synchronization is important.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide an improved data transmission which enables improved synchronization and message transmission in frequency hopping systems.